Wind turbine rotor blades of the type illustrated in FIG. 1 often have a length in excess of 30 meters and currently weigh in the region of 5-15 tonnes, the weight of the blades is likely to increase in the future. Rotor blades are, therefore, manufactured from lightweight, yet strong, materials such as composites e.g. CFRP, GFRP. As in any commercial enterprise, it is necessary to have an efficient manufacturing process to maximise output of the rotor blades. Consequently, composite blades are typically laid up in a mould directly using sheets of material having predetermined, orientation of fibres which are generally woven together and pre-impregnated with resin. Such sheets are commonly referred to as “pre-preg” materials and are cut to the correct shape and layered up within the mould to build up part of an outer shell of the rotor blade. Each shell, or part thereof, is transferred, in its mould to an oven for curing at an elevated temperature.
FIG. 2 illustrates an example rotor blade mould and indicates, with a dashed outline, a region of particularly complex geometry where significant changes in curvature need to be achieved in the shell. Such complexity in the three dimensional geometry of the shell causes difficulty when using sheets of pre-impregnated cloth as it is necessary to avoid folds and wrinkles in the cloth. Folds and wrinkles cause fibres to be misaligned and to be bent or even broken and, as such, are particularly detrimental to the structural integrity of the finished shell component.
This region of complex curvature is also a region of significant and complex loading in an operational rotor blade. In particular, edgewise loads that are experienced along a trailing edge of the blade are transmitted through the section of the blade where the point of maximum chord is transformed to the approximately circular root portion (see FIG. 1) that attaches to the rotor hub (not shown). Careful selection of fibre orientation in such regions can significantly enhance the strength of the blade without increasing the weight of the blade.
It is, therefore, desirable to develop a method of manufacture whereby the orientation of fibres is optimised, the weight of the component is minimised and the problems outlined above are overcome.